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Why do we use disc brakes in front and drum brakes in rear?


Disk brakes have a higher overall capacity, but drum brakes are cheaper and take less hydraulic force to activate. When you stop your car most of the braking is done by the front brakes because of weight transfer (how you feel pressed towards the front when you hit the brakes). These two facts are why you usually see a mixture of the two in this manner.

 

Drum brakes have less hardware since everything is included near the hub and there's no caliper bracketry. Drum brakes also let you have a bit smaller master cylinder (at the pedal) because they don't take as much initial pressure to apply as disk brakes. However, they're less capable of stopping overall and they are more susceptible to warping and brake fade due to high temperatures than disk brakes. They're worse at cooling, and more likely to need cleaning and adjustment for optimal operation than disk brakes because a lot of the dust from brake shoes stays inside the drum. Also, the hand brake which is applied on the rear axle is easier to add to a drum brake than to the inside of the hub of a disk brake. 

Disk brakes require more hardware and higher overall hydraulic pressure to apply. But they cool better, have better overall stopping power and are less susceptible to warping or brake fade with temperature. So they are better suited for the front axle.

 

Most cars have disc brakes only at front and a drum brake at the rear. Some of the cars offers disc brakes at front and rear as well, but only in their top variants. So the question arises what is so special about disc brakes and if at all manufacturers offer disc, why they put it on front wheel?

The weight distribution in a car is broadly in the ratio of 60% front and 40% rear, in an unloaded condition. As all the seats get occupied, this ratio broadly can become 50:50. This also means that the braking forces required at front and rear should be same. But actually when we brake, because of a moment (couple) coming into picture, weight get transferred to the front wheel and the effective weight of front wheel increases. Making it necessary to have a more efficient braking system installed at front than at rear.

 

So which is better, disc brake or drum brake?

When we brake, the majority of the kinetic energy changes into heat because of friction between the brake pads and disc drum, which bring the vehicle to rest. The heat generated has its own impact on the characteristic of the friction material in this case the brake pads.

The brake pads under influence of excessive heat tends to lose (fade) their frictional characteristic, which can increase the stopping distance. The only way out is to remove the heat as soon as it is generated.

The disc dissipates heat more effectively as its surface is open and expose to direct air unlike Drum, which is enclosed, as the name suggests. To further improve the heat dissipation from disc holes are provided. This allows better air circulation helping it function further more efficiently.